Low-loss taps are essential if optical fibers are to be used in multiple-access, local area distribution networks. At each node in such networks, a tap must extract a portion of the through-signal. This, however, has the effect of significantly reducing the level of the remaining signal. For example, if one-quarter of the incident signal is extracted, then there is a 1.25 db loss in the remaining through-signal which severely limits the number of nodes that can be placed along the network. Simultaneously, the tap must also couple a local transmitter to the network. If the coupling coefficient of the second coupler is made low in an effort to reduce the loss along the through-path, the efficiency with which the local transmitter is coupled to the network is similarly reduced. This, in turn, increases the output power requirements of the transmitter.
These various conflicting requirements are satisfied by using active taps in which a portion of the power extracted at each node is amplified in a parallel path and then reinjected into the network. This makes it possible to reduce the losses at each node to zero. (See, for example, the article by A. Albanese entitled "Fail-Safe Nodes for Lightguide Digital Networks" published in the February 1982 issue of the Bell System Technical Journal, Vol. 61, No. 2; and U.S. Pat. No. 4,310,217.) To be commercially practical, however, the tap must also be inexpensive, and conveniently packaged for easy field installation.